ENGINEERING GRAPHICS

SCALES

• Introduction

• Representative Fraction

• Types of Scales

INTRODUCTION

Usually the word scale is used for an instrument

used for drawing straight lines.

But actually in Engineer’s language scale means

the proportion or ratio between the dimensions

adopted for the drawing and the corresponding

dimensions of the object.

INTRODUCTION

I.S. have recommended the following standard

scales.

Full Scale (1:1)

Reduced Scale

1 : 2 1 : 2.5 1 : 5

1 : 10 1 : 20 1 : 50

1 : 100 1 : 200

Enlarged Scale

10 : 1 5 : 1 2 : 1

INTRODUCTION

INTRODUCTION

REPRESENTATIVE FRACTION (R.F.)

The ratio of the size of the element in the drawing

to the size of the same element in the object is

called the Representative Fraction (R.F.).

REPRESENTATIVE FRACTION (R.F.)

Example 1

If 1 cm length of drawing represents 5 m length of

the object, then in engineering scale it is written as

1 cm = 5 m and in graphical scale it is denoted by

REPRESENTATIVE FRACTION (R.F.)

Example 2

If a 5 cm long line in the drawing represents 3 km

length of a road, then in engineering scale it is

written as 1 cm = 600 m and in graphical scale it is

denoted by

REPRESENTATIVE FRACTION (R.F.)

Example 3

If a gear with a 15 cm diameter in the drawing

represents an actual gear of 6 mm diameter in

graphical scale, it is expressed by

TYPES OF SCALES

1.

Mechanical Engineers’

scale

Architects’ scale

Civil Engineers’ scale

2.

Plain scale

Diagonal scale

Comparative scale

Vernier scale

Scale of Chords

Isometric scale

Scales are classified in two different manner as under:

TYPES OF SCALES

These scales are 300

mm long and each unit

is sub-divided.

Mechanical Engineers

generally use following

scales.

1:1 1:2 1:2.5 1:5

2:1 5:1

Mechanical Engineers’ scale

TYPES OF SCALES

Architects are required to take very small R.F. since buildings are comparatively very big as compared to drawing paper size.

Only the first main division of the architects’ scale is sub-divided.

Architects’ scale

TYPES OF SCALES

Civil Engineers dealing

with road maps and

survey maps are

required to take very

very small R.F..

These scales are sub-

divided on their entire

lengths.

Civil Engineers’ scale

PLAIN SCALES

Plain scales read or measure upto two units or a unit and its

sub-division, for example centimeters (cm) and millimeters

(mm).

When measurements are required upto first decimal, for

example 2.3 m or 4.6 cm etc.

It consists of a line divided into number of equal main parts

and the first main part is sub-divided into smaller parts.

PLAIN SCALES

Example

A 3 cm long line represents a length of 4.5 meters.

Extend this line to measure upto 30 meters and

show on it units of meter and 5 meter. Show the

length of 22 meters on this line.

Construction:

Draw a straight line of 20cm length and divide into 6 equal parts.

Divide again first part into 5 equal parts. Give numbers as shown. To

represent 22 meters, take 4 main parts to represent 20 meters and 2

small parts to represent 2meters. Give names as A and B so that the

distance between A and B is 22 meters as shown.

Note: Assume height of the plain scale as 1 cm.

DIAGONAL SCALES

Diagonal scales are used to read or measure upto three units.

For example: decimeters (dm), centimeters (cm) and millimeters (mm) or

miles, furlongs and yards etc. This scale is used when very small

distances such as 0.1 mm are to be accurately measured or when

measurements are required upto second decimal.

For example: 2.35dm or 4.68km etc.

Small divisions of short lines are obtained by the principle of diagonal

division, as explained below:

Principle of diagonal scale: To divide a given line AB into small divisions

in multiples of 1/10 its length for example 0.1AB; 0.2AB etc.

DIAGONAL SCALES

Example

An area of 144 sq. cm on a map represents an area

of 9 sq. km on the field. Find the R.F. of the scale

for this map and draw a diagonal scale to show

kilometers, hectometers and decameters and to

measure upto 5 kilometers. Indicate on the scale a

distance of 3 kilometers, 5 hectometers and 6

decameters or 3.56km.

Construction: Draw a line AB of 20 cm and construct a rectangle on it, by taking AD

5cm as shown. Divide AB into 5 equal parts and number them from

second part starting with 0 to 4 towards right side to indicate kilometers

(km). Divide 0A into 10 equal parts, each part represents a hectometer

(hm). Divide AD into 10 equal parts, each part represents one decameter

(dam). Join diagonals as shown.

To mark 3.56km, take it as sum of 3.50km and 0.06km. On the plain

scale take 3.5km and on the diagonal at 5 upto 6 parts diagonally which

is equal to 0.06km, giving a total of 3.56km as shown by MN.

Note: Assume the height of the diagonal scale AD as 5cm for dividing it

into 10 equal parts conveniently.

ISOMETRIC SCALES

A scale is now constructed by stepping off true measurements along line 'AB1' which is a true length line.

The measurements are then transferred back to line 'AB' to get a smaller scale, in this case an isometric scale.

Lines drawn using the isometric scale are approximately 80% of true size. This scale is usually marked off on a piece of paper and used to step off the foreshortened measurements along the projection of axes lines and lines parallel to them.

Lines parallel to the projection of axes are known as isometric lines.

Lines which are not parallel to theses axes are known as non-isometric lines.

It is important to note that you can only use the scales on isometric lines.